When the full operating voltage is immediately applied to start an induction motor, the current during start-up can be six or more times the normal operating current. Therefore the power consumed during start-up may be considerable. In addition, the resulting starting torque can be three times normal, causing a decrease in the life of mechanical components, such as bearings, clutches and belts coupled to the motor.
Solid state motor controllers have been employed for sometime to start large electric induction motors. Many of these motors are three phase devices in which electric current for each phase must be switched in order to provide full motor control. Solid state motor controllers often provide a "soft start" in which the voltage applied to the motor is gradually increased, rather than applying the full operating voltage to the motor in a single step function. The soft start reduces the electrical current drawn by the motor during the starting and thereby reduces power consumption. Such controllers also sense fault conditions in the motor, such as a stall, and take measures to either correct the fault or to prevent damage to the motor.
In prior art systems, electronic logic circuits operated thyristors which controlled the electricity applied to the motor. All of the controller components were incorporated into a single housing to which the electrical power and the motor were connected. In providing a full line of controllers for motors of various sizes, i.e. different voltage and current ratings, various controllers having the same control logic sections but different types of thyristors had to be fabricated.
If these motor controllers failed in the field, typically because one or more of the thyristors burned out, the entire assembly, including the control electronics, had to be replaced. This dramatically increased the cost of such replacement. Furthermore, if the motor is replaced with one having a larger power rating, the controller may also have to be entirely replaced.